[aprssig] Handling incoming messages for different SSID

Lynn W. Deffenbaugh (Mr) ldeffenb at homeside.to
Sun Nov 13 16:04:49 CST 2011

On 11/13/2011 4:27 PM, Georg Lukas wrote:
> Should the application notify the user of incoming messages to all their
> SSIDs (DO1GL*)?  Only of messages to the configured SSID (DO1GL-5)? Or
> configured SSID and no-SSID (DO1GL-5, DO1GL)?

Yes, but I'm with you, I cannot put my eyes on the actual spec that says 
that.  I remember reading something about a station should "read" (their 
word, not mine) any message for the same callsign as the station but 
should only acknowledge messages addressed to the specific -SSID of the 
station.  Maybe it's Kenwood that set that standard, but I know it's 
written down somewhere in the pages and pages of "additional" specs.

> Should there be a difference in display of such messages? Right now, I
> do not display the user's own callsign due its redundancy.

Given that I support various types of user-enabled "eavesdropping" in 
APRSISCE/32, I already display the callsign-SSIDs of both ends of a 
conversation.  This is no different than watching the raw message 
packets on the air or via APRS-IS, I just format them nicely.  In fact, 
I leave it up to the user if they want to receive all of their own 
messages (regardless of -SSID), but I default that option to ON.

> I suppose, the application should only send ACKs for messages directed
> at the configured callsign-ssid to avoid duplication problems. Is that
> correct?

Yes only ack the specific callsign-SSID of your instance.  From aprs101.pdf:

> The receiving station will only acknowledge messages addressed to itself,
> and not any messages received which were addressed to any group callsign.
>  Note: The receiving station will acknowledge all messages addressed to
> itself, even if it is operating in an Alternate Net (see Chapter 4: 
> APRS Data
> in the AX.25 Destination and Source Address Fields).

> Should the application strip the SSID from the destination callsign in
> outgoing messages?

No, any message sent is sent from a specific callsign-SSID to a specific 
callsign-SSID.  Otherwise, you may never get an ack causing excessive 

> Where can I find documentation / specification regarding the above
> questions?

Read, study, sleep on, and re-read aprs101.pdf.  There's lots of 
subtleties in there!  Like:

> The receiving station will read all messages with the Addressee field 
> set to
> ALL, QST or CQ.

So, in addition to "reading" all messages to the base callsign, you're 
also supposed to "read" messages to those three additional group names 
(and support local user-configurable additional ad-hoc message groups).  
Kenwood adds their own (not sure what it is right now) and I also listen 
to APRSIS-32 and APRSIS-CE as pre-defined "special" group names in 
keeping with the Kenwood example.

Lynn (D) - KJ4ERJ - Author of APRSISCE for Windows Mobile and Win32

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